The present invention relates to simultaneously reading and writing on an optical medium such as an optical tape.
Direct Read After Write (DRAW) has been recognized as a design option for optical recording systems since the earliest introduction of the technology. (See Marchant, Optical Recording: a Technical Overview, Addison Wesley, 1990, p304.) As illustrated in FIG. 1, a prior art optical head 10 is schematically shown. A record laser 12 directs a beam of light establishing a beam path through an objective lens 14 which is focused on a recording spot 16 on a moveable optical medium 18 that is moving in direction 25 relative to the optical head. Also, a read laser 20 establishes and directs a beam of light through a beam splitter 22 and the objective lens 14 onto the surface of the moveable optical medium 18 to form a read spot 24. This beam of light is reflected back to the beam splitter 22 which directs the light beam to a detector 26 shown as a photodiode.
More particularly, the beam from the read laser 20 is unmodulated and is focused on the moveable medium 18 downstream from the recording spot. Reflection of the read laser beam from the medium is modulated by the presence of marks on the recording medium newly recorded by the record laser beam. The detector 26 reconstructs the recorded data for comparison with the data just processed for recording. This is why this process is called Direct Read After Write, or DRAW. DRAW provides nearly instantaneous verification of recorded data reliability, eliminating the latency encountered with conventional, second-pass verification.
In optical recording, there must be some level of verification that the recording medium will support error free recording and reproduction. When optical tape is the medium of choice, there is also a requirement that the recording system can record and read bi-directionally, that is, when the tape is either moving in a forward or reverse direction. A problem with optical tape recording systems is that existing DRAW heads do not have this functionality.
It is an object of the present invention to provide apparatus which provides DRAW functionality when the optical recording medium is moveable bi-directionally in either forward or reverse directions.
This object is achieved by an apparatus for simultaneously recording and reading information on a medium moveable in forward and reverse directions, comprising:
a) means including a first record laser and establishing a first beam path for recording information on the moveable medium;
b) means including at least a second read laser and at least a first and second read-out detector respectively associated with the forward and reversed directions of the recording medium, said means establishing two beam paths which direct the laser light to positions forward and rearward from where the first laser beam illuminates the medium and such read laser beams are reflected respectively to the first and second detectors; and
c) means for selecting either the first or the second read-out detectors and for processing information read by such selected detector so that as the medium is moving in the forward direction, the data are read from the first detector and when the medium is moving in the second direction, the data are read from the second detector.
The present invention has as an advantage that it permits the DRAW read beam to be focused at a position downstream from the recording beam, with respect to scanning of the medium during recording. In the prior art, if the media velocity is reversed, as in serpentine recording on optical tape, the DRAW beam is sometimes positioned on the upstream side of the recording beam, and therefore cannot detect the recorded pattern.
A feature of the present invention is that it provides verification without requiring prior art methods of pre-scanning for media defects or multiple-pass readout for data verification.